Financial risk management objectives and policies - Risk Exposure (Details) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Currency and interest rate risk \| Fair value hedges of foreign currency and interest rate risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedge ineffectiveness (loss)/gain	£ (3)	£ 0
Fair value (loss)/gain arising on hedging instruments deferred to OCI	0	0
Fair value amounts reclassified to profit and loss	£ 0	£ 0
Weighted average interest rate		0.00%
Weighted average FX rate	1.17	0
Currency and interest rate risk \| Hedging Instruments \| Fair value hedges of foreign currency and interest rate risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, liabilities	£ (15)	£ 0
Notional amount of hedged items	£ 0	£ 0
Notional amount of hedging instruments	500	0
Change in fair value of hedging instrument (loss)/gain	£ (7)	£ 0
Currency and interest rate risk \| Non-Derivative Hedging Instrument \| Fair value hedges of foreign currency and interest rate risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, liabilities	0	0
Currency and interest rate risk \| Hedged Items \| Fair value hedges of foreign currency and interest rate risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount of hedged items	500	0
Change in fair value of hedged items gain/(loss)	4	0
Currency risk \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedge ineffectiveness (loss)/gain	(3)	3
Fair value (loss)/gain arising on hedging instruments deferred to OCI	(35)	(43)
Fair value amounts reclassified to profit and loss	£ 58	£ 44
Weighted average interest rate	4.45%	4.43%
Weighted average FX rate	1.14	1.13
Currency risk \| Net investment hedges of foreign currency risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedge ineffectiveness (loss)/gain	£ 0	£ 2
Fair value (loss)/gain arising on hedging instruments deferred to OCI	(3)	108
Fair value amounts reclassified to profit and loss	£ 0	£ 0
Weighted average interest rate	5.24%	4.62%
Weighted average FX rate	1.24	1.23
Currency risk \| Hedging Instruments \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, liabilities	£ (56)	£ (17)
Notional amount of hedged items	£ 0	£ 0
Notional amount of hedging instruments	1,400	1,250
Change in fair value of hedging instrument (loss)/gain	£ (5)	£ (29)
Currency risk \| Hedging Instruments \| Net investment hedges of foreign currency risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, assets	20	48
Notional amount of hedged items	£ 1,285	£ 1,874
Notional amount of hedging instruments	1,285	1,874
Change in fair value of hedging instrument (loss)/gain	£ (3)	£ 110
Currency risk \| Non-Derivative Hedging Instrument \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, liabilities	0	0
Currency risk \| Non-Derivative Hedging Instrument \| Net investment hedges of foreign currency risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, liabilities	(244)	(835)
Currency risk \| Hedged Items \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount of hedged items	1,400	1,250
Change in fair value of hedged items gain/(loss)	2	32
Currency risk \| Hedged Items \| Net investment hedges of foreign currency risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount of hedged items	0	0
Change in fair value of hedged items gain/(loss)	3	(108)
Interest rate risk \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedge ineffectiveness (loss)/gain	0	0
Fair value (loss)/gain arising on hedging instruments deferred to OCI	0	0
Fair value amounts reclassified to profit and loss	£ 0	£ 0
Weighted average interest rate	4.96%	0.00%
Weighted average FX rate	0	0
Interest rate risk \| Hedging Instruments \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, liabilities	£ 0	£ 0
Notional amount of hedged items	£ 0	£ 0
Notional amount of hedging instruments	428	0
Change in fair value of hedging instrument (loss)/gain	£ 0	£ 0
Interest rate risk \| Non-Derivative Hedging Instrument \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Hedging instrument, liabilities	0	0
Interest rate risk \| Hedged Items \| Cash Flow Hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount of hedged items	428	0
Change in fair value of hedged items gain/(loss)	£ 0	£ 0

The average price of a hedging instrument. [Refer: Hedging instruments [domain]]

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The average rate of a hedging instrument. [Refer: Hedging instruments [domain]]

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

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No definition available.

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The gain (loss) on the change in fair value of the hedging instrument used as a basis for recognising hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The gain (loss) recognised on designation of a financial instrument, or a proportion of it, as measured at fair value through profit or loss, because a credit derivative is used to manage the credit risk of that financial instrument. [Refer: Credit risk [member]; Derivatives [member]; Classes of financial instruments [domain]]

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The gains (losses) recognised in profit or loss for liabilities measured at fair value that are attributable to the change in unrealised gains or losses relating to those liabilities held at the end of the reporting period. [Refer: At fair value [member]]

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The amount of a hedging instrument, recognised as an asset. [Refer: Hedging instruments [domain]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The amount of a hedging instrument, recognised as a liability. [Refer: Hedging instruments [domain]]

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The nominal amount of a hedging instrument. [Refer: Hedging instruments [domain]]

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The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

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